A specific application of the invention is in the use of a photodisruptive laser for defining a resection plane of a corneal layer to create corneal flap in ophthalmic surgical procedures for vision error correction. Vision impairment can occur for many reasons, and be the result of many causes. One common cause for vision impairment results from a defective condition of the eye which occurs when the refractive characteristics of the cornea do not cause parallel rays of light to focus on the retina. When the eye is at rest, and the rays of light focus in front of the retina, the condition is known as myopia (i.e. nearsightedness). On the other hand, when the rays of light focus behind the retina, the condition is known as hypermetropia or hyperopia (i.e. farsightedness). Both myopic and hyperopic conditions result in varying degrees of vision impairment. In most cases the conditions are correctable.
Eyeglasses or contact lenses are commonly used to correct myopic or hyperopic conditions. For various reasons, however, many persons who suffer with these conditions prefer not to wear eyeglasses or contact lenses. Alternative ways to correct these conditions include known surgical procedures for reshaping the cornea in various ways that are effective in changing its refractive characteristics. For example, in U.S. Pat. Nos. 4,665,913 and 4,669,466 to L'Esperance, a laser system is described which photoablates corneal tissue from the anterior surface of the eye. Another procedure is described in U.S. Pat. No. 4,988,348 to Bille, whereby corneal tissue is first removed to correct vision, and then the newly created surface is smoothed.
Rather than remove and reshape portions of the anterior portion of the eye to correct refractive defects, other procedures have been developed using a technique called intrastromal photodisruption for removing internal stromal tissue. An example of such a procedure is described in U.S. Pat. No. 4,907,586 to Bille et al. Another example of a procedure for removing stromal tissue is the procedure described in U.S. Pat. No. 6,110,166 to Juhasz. In this procedure, an anterior corneal layer can be defined by using a laser to create a series of overlapping photodisrupted areas. The surgeon then separates the corneal layer by lifting it, to gain access to the underlying corneal tissue, which is changed through photoablation. The corneal layer is then repositioned on the cornea.
The photodisruption procedure involves removal of tissue in a stroma in a cornea of an eye using pulsed laser beam which is sequentially focused to individual spots at a plurality of locations in the stroma. Each focus spot has a finite volume, rather than being a single point. Further, each spot has a central point at approximately the center of the finite volume. Photodisruption of stromal tissue occurs at each spot where the beam is focused when fluence is above the threshold value and the volume of stromal tissue disrupted at each spot is approximately equal to the volume of the spot. The amount of tissue damage is dependent on how much the fluence exceeds the threshold value An optimal fluence value exists for a given separation between photodisruption spots to achieve the best surgical result. For example, if the fluence is below the optimal value, then it is difficult to lift the flap. If the fluence is above the optimal value, then an excessive amount of gas is produced during the photodisruption process creating opacity in the cornea, thus complicating the next step of vision correction procedure, photoablation. Clinical studies show that noticeable differences in outcomes occur when fluence varies +/−10%. Consequently, it is important to have a uniform distribution of the fluence between photodisruption points.
Such a pulsed laser syste, (which includes the laser and focusing optics), ideally provides an even fluence distribution across the focal plane, thus providing uniform distribution of the photodisruptive effect. However, the laser systems used in these procedures present the problem of providing nonuniform fluence over a focal plane even when set at a constant energy because of variations of the focal spots in the focal plane. Thus, the variance in fluence distribution may be above the optimal value at some points in the focal plane and below the optimal value at other points in the laser focal plane. This, in turn, results in nonuniform distribution of photodisruption in the focal plane. One reason for the fluence variance is that the optic that the laser is focused through, although generally uniform, contains imperfections and small variations resulting in aberrations in the beam. Aberrations generally change the spot size in the focal plane. By correcting energy, the present invention minimizes the fluence variance at each point in the focal plane where the spot size varies because of aberrations in the laser beam.
U.S. Pat. No. 6,287,299 describes a method of monitoring fluence from focus spot to focus spot by directing a portion of the laser beam energy to a fluence monitoring device to provide a picture of fluence distribution over a curved surface in overlaying pattern. Fluence is controlled by controlling the number of pulses irradiating a single point and by overlaying the spots. It is essential for that method to have multiple pulses irradiating the same point in X/Y plane. However, the '299 patent fails to address the issue of correcting fluence variance due to discrepancies in the focusing optics. Furthermore, the method is not useful for single pulse photodisruption in real-time surgical settings and for high numerical aperture focusing optics when the space between the focusing lens and focal plane is very limited.
Herein, the inventors present a method and apparatus for overcoming the disadvantages of the prior art.